1. Field of the Invention
This invention relates to pool-type nuclear reactors and more particularly provides an arrangement for substantially separating the hot and cold coolant plena in a pool-type liquid metal cooled fast breeder reactor.
2. Description of the Prior Art
In a pool-type nuclear reactor a large reactor vessel contains the major components, such as the reactor core, coolant pumps and main heat exchangers, within a pool of liquid coolant such as sodium. Generally, the coolant is pumped from a cold plenum into and through the core, and discharged into a hot plenum. From the hot plenum the coolant flows through the heat exchangers, transferring heat energy to another coolant, typically for the ultimate purpose of electric power generation, and is discharged back to the cold plenum.
A major component of such reactors has been a structure, known alternatively as a plenum separator, reactor jacket, inner tank, primary tank, insulated internal tank or internal thermal liner, which separates the hot coolant in the hot plenum from the reactor vessel wall so as to alleviate thermal transients and stresses. This component also serves to somewhat separate the hot and cold plena in some configurations. In typical existing pool reactor designs the plenum separator is a cylindrical shell which is arranged to place low temperature sodium coolant in an annular region in contact with the reactor vessel wall. In most arrangements, the free or upper surface of this cold coolant changes in elevation during reactor operation, and particularly at start-up and shutdown, by up to eight feet as a result of the pressure differentials between the plenums necessary to circulate the coolant through the primary heat exchangers. Such cycling is undesirable as it results in substantial thermal transients on the vessel wall and the plenum separator. An arrangement which alleviates the elevation change is that of the Soviet BN-600 plant which pumps coolant from the cold pool upwardly through an annulus adjacent the vessel wall and into the top of the hot plenum. While this arrangement alleviates the elevational fluctuations, it requires a forced pumping and the coolant passing through the annulus is not available for core cooling, presenting an overall loss of efficiency. Additional thermal stress concerns are also raised in the BN-600 and other pool reactor arrangements as the lower support structure, which typically supports the weight of the core components, is directly exposed on its top surface to the hot coolant and on its bottom surface to the cold coolant. This temperature differential can range up to approximately 300+ F. Such arrangements also typically require large amounts of under-sodium insulation in the region of the plenum separator, the long-term operational characteristics of which are not totally certain.
Accordingly, it is desirable to provide a plenum separator arrangement which alleviates the above and other thermal transient and stress concerns while providing acceptable reactor system efficiency.